The present invention relates to a novel aqueous slurry of rare earth hydroxide particles and to a method for the preparation thereof. More particularly, the invention relates to an aqueous slurry of rare earth hydroxide particles which can be used as a sintering aid or an additive in the preparation of a sintered ceramic body as well as to a method for the preparation of the slurry.
As is well known, certain rare earth compounds are used as a sintering aid or an additive ingredient in the preparation of various kinds of sintered ceramic bodies. When to be used in such applications, the particles of the rare earth compound are required to satisfy various requirements including not only fineness of the particle size but also good sintering behavior, high reactivity with other ingredients of the ceramic composition, excellent dispersibility in ceramic blend for sintering, and so on. In this regard of satisfying various requirements, the most widely employed rare earth compound in the ceramic preparation is a rare earth oxide in the form of a fine powder, which is prepared by several methods known in the art. For example, fine particles of an oxidatively decomposable compound of a rare earth element such as oxalates, carbonates and hydroxides are precipitated in a solution of a soluble rare earth compound and the precipitates are collected and calcined in an oxidizing atmosphere to be converted into a rare earth oxide powder. Alternatively, fine particles of a rare earth oxide can be prepared by grinding relatively coarse oxide particles in a suitable grinding machine. The rare earth oxide particles prepared by these typical prior art methods, however, are not always very satisfactory relative to the particle size because the average particle diameter of the oxide particles obtained by these methods can rarely be smaller than about 1 xcexcm. Accordingly, it is desired in recent years to prepare particles of a rare earth compound having a finer and finer particle size in order to comply with the trend of the powdery compositions as the principal ingredient of the ceramic formulation toward a finer and finer particle size.
The present invention accordingly has an object, in view of the above described problems and disadvantages relative to powders of a rare earth compound, to provide particles of a rare earth compound or, in particular, rare earth hydroxide having an extremely small diameter of primary particles not exceeding 150 nm in the form of an aqueous slurry or dispersion in which the particles are little subject to settling with good dispersibility and stability as well as a method for the preparation thereof.
Thus, the present invention provides an aqueous slurry of rare earth hydroxide particles which comprises: a powder of a rare earth hydroxide of which the primary particles have a diameter not exceeding 150 nm; and water as the dispersion medium of the rare earth hydroxide particles, and which has an electric conductivity not exceeding 2 mS/cm.
The method of the present invention for the preparation of the above defined aqueous slurry of particles of a rare earth hydroxide comprises the steps of:
(a) mixing a first aqueous solution of a rare earth compound and a second aqueous solution of a basic compound, if necessary, in the presence of a surface active agent to precipitate rare earth hydroxide particles in an aqueous medium;
(b) collecting the rare earth hydroxide particles by separating from the aqueous medium to give a cake of the rare earth hydroxide particles;
(c) repeatedly washing the cake of the rare earth hydroxide particles with deionized water until the washing has an electric conductivity not exceeding 3 mS/cm; and
(d) dispersing the rare earth hydroxide particles in deionized water as a dispersion medium.
FIG. 1 is a transmission electron microscopic photograph of the holmium hydroxide particles in the aqueous slurry prepared in Example 1.
FIG. 2 is a transmission electron microscopic photograph of holmium hydroxide particles for the same view field as in Photo 1 in a larger magnification.
The above defined inventive method for the preparation of an aqueous slurry of rare earth hydroxide particles is applicable to any rare earth element including yttrium and the elements having an atomic number in the range from 57 to 71 without particular limitations.
The rare earth compound used as the starting material in step (a) of the inventive method is not particularly limitative provided that the compound has good solubility in water including chlorides, sulfates, nitrates, aliphatic ammonium salts, salts of sulfate esters, salts of phosphate esters, sulfonates, carboxylates and ether compounds, of which salts with an inorganic acid, i.e. hydrochloric, sulfuric and nitric acids, are preferable and nitrates are more preferable.
The precipitation reaction of the rare earth hydroxide particles by mixing an aqueous solution of the rare earth compound and an aqueous solution of a basic compound is conducted optionally but preferably in the presence of a surface active agent which can be contained in either of the aqueous solutions to be mixed together although it is preferable that the surface active agent is contained in the aqueous solution of the rare earth compound. The surface active agent used here is not particularly limitative relative to the ionic nature thereof including cationic, anionic, non-ionic and amphoteric surface active agents without particular problems.
The cationic surface active agent mentioned above is exemplified by quaternary ammonium compounds such as long chain-alkyl trimethylammonium chlorides and the like. The anionic surface active agent is exemplified by sodium alkylbenzenesulfonates, sodium laurylsulfate, sodium lauryl ether sulfate and the like. The non-ionic surface active agent is exemplified by polyoxyalkylene alkyl ethers, e.g., polyoxyethylene alkyl ethers, and polyoxyalkylene alkylphenyl ethers, e.g., polyoxyethylene nonylphenyl ether. The amphoteric surface active agent is exemplified by betaine lauryl dimethylamino acetate.
Although the ionic nature of the surface active agent is not particularly limitative as is mentioned above, it is preferable to use a cationic surface active agent in respect of the high effectiveness. Various commercial products of cationic surface active agents are available and can be used in the inventive method including those sold under the trade names of Armac C (a product by Lion Akzo Co.), which is an alkyl amine salt-type surface active agent. The amount of the surface active agent contained in the aqueous solution or solutions is in the range from 0.01 to 10 g or, preferably, from 0.1 to 10 g or, more preferably, from 0.5 to 5 g per mole of the rare earth element in the rare earth compound. When the amount of the surface active agent is too small, the desired effects to be obtained thereby cannot be fully accomplished as a matter of course while no further advantages can be obtained by increasing the amount thereof to exceed the above mentioned upper limit.
The basic compound, of which an aqueous solution is mixed together with the aqueous solution of the rare earth compound in step (a) of the inventive method, is also not particularly limitative but is preferably an inorganic hydroxide such as sodium hydroxide, potassium hydroxide and ammonium hydroxide, i.e. ammonia water, of which ammonia water is preferred.
The first aqueous solution, which contains the rare earth compound, and the second aqueous solution, which contains the basic compound, can be mixed together in different ways including (1) addition of the second solution to the first solution, (2) addition of the first solution to the second solution and (3) concurrent addition of the first and second solutions to a volume of water as a receiving bath. The surface active agent can be dissolved in either one, two or all of the first solution, the second solution and the receiving bath, when used.
The concentrations of the rare earth compound and the basic compound in the respective solutions to be mixed together in step (a) of the inventive method should be selected in such a way that the aqueous slurry of the rare earth hydroxide particles formed by the admixture of the solutions contains the hydroxide particles in an amount not exceeding 0.2 mole per liter. This is because, when the content of the hydroxide particles in the aqueous slurry is too high, the aqueous slurry exhibits gel-like consistency so that difficulties are encountered in agitation of the slurry with a stirrer.
The amount of the basic compound contained in the above mentioned second aqueous solution should be sufficient to completely precipitate the rare earth element in the form of a hydroxide of the element or, preferably, in the range from 1.0 to 1.5 times of the stoichiometrically equivalent amount for complete precipitation. When the amount of the basic compound is too small, a portion of the rare earth element remains unprecipitated and is contained in the precipitation medium resulting in an economical disadvantage due to a decrease in the yield of the hydroxide particles while no further improvement can be obtained in the yield of the hydroxide particles even by increase the amount of the basic compound to exceed the above mentioned upper limit resulting also in an economical disadvantage.
The reaction temperature for the precipitation reaction of the rare earth hydroxide particles is not particularly limitative but, preferably, should not be higher than 50xc2x0 C. or, more preferably, should be room temperature or lower. When the reaction temperature is too high, a disadvantageous increase is caused in the diameter of the primary particles of the hydroxide precipitates formed by the reaction.
The mixing rate of the two aqueous solutions in step (a) of the inventive method is also not particularly limitative and can be selected in such a way that the mixing procedure of the solutions is completed within several minutes under agitation. It is optional that the aqueous slurry of the rare earth hydroxide particles formed by the reaction is continuedly agitated for several minutes after completion of the admixture of the solutions to effect so-called aging of the precipitates.
The precipitates of the rare earth hydroxide thus formed in the precipitation medium are then, in step (b), collected by separating from the precipitation medium in a known solid-liquid separating method such as centrifugation and filtration to give a cake of the precipitates.
In step (c) of the inventive method, the cake of the rare earth hydroxide particles is washed with deionized water to remove the electrolytes adhering to or contained in the hydroxide particles as completely as possible, if necessary, by repeating washing which can be carried out by a known procedure without particular limitations. It is essential that the procedure of washing with water is continued or repeated until the washing separated from the precipitates has an electric conductivity of 3 mS/cm or lower followed by recovery of the hydroxide cake. If the washing procedure is terminated before the electric conductivity of the washing decreases to 3 mS/cm, the hydroxide particles eventually exhibit strong agglomeration to adversely affect the dispersibility of the particles in an aqueous medium.
The last step of the inventive method for the preparation of an aqueous slurry of the rare earth hydroxide particles is for dispersion of the above obtained wet cake of the rare earth hydroxide particles in deionized water by a known procedure in which, for example, the wet cake is introduced into an appropriate volume of deionized water contained in an agitation tank and vigorously agitated therein by using a suitable stirrer machine to be dispersed in the form of an aqueous slurry. Homogenizers are preferred as the stirrer machine when an aqueous slurry of high dispersion should be obtained within a short time. The content of the rare earth hydroxide particles in the aqueous slurry is desirably selected, though dependent on the particularly intended application of the aqueous slurry, in the range from 1 to 20% by weight or, preferably, from 5 to 20% by weight calculated as the rare earth oxide. When the amount of the hydroxide particles is too large relative to the amount of the water as the dispersion medium, dispersion of the particles is sometimes in complete resulting in an inhomogeneous aqueous slurry while, when the amount of the hydroxide particles is too small, an economical disadvantage is caused due to the unduly large volume of the aqueous slurry under handling. When the cake of the rare earth hydroxide particles has been fully washed as specified above in terms of the electric conductivity of the washing, the thus obtained aqueous slurry of the hydroxide particles usually has an electric conductivity not exceeding 2 mS/cm.
The rare earth hydroxide slurry prepared in the above described manner can be used as a component or as a sintering aid of a ceramic molding composition since the rare earth hydroxide particles in the aqueous slurry have good dispersibility with particles of other ceramic materials such as silicon nitride, aluminum nitride, zirconia, barium titanate and others to give a very uniform ceramic molding composition which is molded and subjected to sintering at a temperature of, for example, 600 to 1600xc2x0 C. or higher to give a sintered ceramic body having a high sintering density and a very uniform and dense structure. It is desirable here that the particles of the base ceramic material, to which the rare earth hydroxide slurry of the invention is added, has an average particle diameter not exceeding 1 xcexcm in order to fully accomplish the advantage to be obtained by the addition of the inventive aqueous slurry of the rare earth hydroxide particles having good dispersibility for the fineness of the particle size.
The adding amount of the inventive aqueous slurry of the rare earth hydroxide as a sintering aid of a ceramic composition should be selected depending on the types of the base ceramic material. When silicon nitride or aluminum nitride is employed as the base ceramic material, for example, the adding amount of the inventive aqueous slurry is in the range from 0.1 to 20% by weight or, preferably, from 0.5 to 10% by weight calculated as the rare earth oxide based on the total amount of the ceramic molding composition.
In the following, particular embodiments of the present invention are described in more detail by way of Examples, which, however, never limit the scope of the invention in any way.